# Calculation of Drag, Wall shear stress, Lift
**URL:** https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329
**Category:** ParaView Support
**Created:** [August 30, 2024, 11:04am UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329 "2024-08-30T11:04:40Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Pranay\_006](https://discourse.paraview.org/u/Pranay_006)
#### Post date: [August 30, 2024, 11:04am UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329/1 "2024-08-30T11:04:40Z")
Hello
I have a simulation around the airfoil the field files generated as .f00000 and a binder file .nek5000. I am trying to see if it is possible to extract surface of the airfoil as shown in the video below or calculate the following:
1. Wall shear stress and the profile from the wall if possible
2. Calculation of lift and drag forces
[How to calculate aerodynamic forces with Paraview (youtube.com)](https://www.youtube.com/watch?v=J944HOj_4b0)
There was a previous discussion of calcuation of Cp vs x/c in the post
[Manipulation of field files - ParaView Support - ParaView](https://discourse.paraview.org/t/manipulation-of-field-files/14774)
Example field files : [Re10000](https://fraunhofer-my.sharepoint.com/:f:/g/personal/sai_pranay_padakanti_iwes_fraunhofer_de/EnkRZbHw09VLvTI6elfo_McBVKEMRE1uTNJuJ2B8EXUJJQ?e=8WC9Od)
Thank you
Pranay
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### Author:  [@Kenneth\_Moreland](https://discourse.paraview.org/u/Kenneth_Moreland)
#### Post date: [August 30, 2024, 12:26pm UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329/2 "2024-08-30T12:26:49Z")
I loaded the data set you posted. What I see is a slab of data with a hole in the middle that I expect is the cross section of your airfoil.
In the video you reference, the speaker extracts a block of the surface to compute the shear-wall stress on. Your data only has one block, so that won’t work. I don’t know if you can direct Nek5000 to write out this block, but you should be able to extract it in ParaView without too much trouble.
While looking at your data, I noticed that your mesh is not well connected. That is, when you have two hexahedra sharing a face, they are not sharing the same points. That is going to mess up ParaView’s neighbor calculations. You can fix this by running the `Clean to Grid` filter.
You can get the polygons around that hole in your data by applying the `Extract Surface` filter (after `Clean to Grid`). After you apply that filter, things will look the same because you are also extracting the outer boundary of the mesh. You can remove the outer boundary that you don’t want by applying the `Extract Cells By Region` filter.
In the `Cells By Region` filter, change the `Intersect With` parameter to `Box`. Then, adjust the planes of the box to be inside the boundary. Check the box that says `Extract intersected`.
Here is a statefile of what I did:
[extract-airfoil-surface.pvsm](https://discourse.paraview.org/uploads/short-url/vdbMYb8d25PF7kKHrJ105tSa3R8.pvsm) (647.4 KB)
With this surface you should be able to follow the video’s instructions of computing normals and calculating the forces.
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### Author:  [@Pranay\_006](https://discourse.paraview.org/u/Pranay_006)
#### Post date: [August 30, 2024, 12:29pm UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329/3 "2024-08-30T12:29:52Z")
Thanks a lot, will look into it.
Appreciate the quick response
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### Author:  [@Pranay\_006](https://discourse.paraview.org/u/Pranay_006)
#### Post date: [August 30, 2024, 12:36pm UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329/4 "2024-08-30T12:36:23Z")
Another question, is it possible to extract wall shear stress around the surface or does it have to come from NEK5000 as we do it in OpenFOAM?
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### Author:  [@Kenneth\_Moreland](https://discourse.paraview.org/u/Kenneth_Moreland)
#### Post date: [August 30, 2024, 1:20pm UTC](https://discourse.paraview.org/t/calculation-of-drag-wall-shear-stress-lift/15329/5 "2024-08-30T13:20:04Z")
Maybe. I don’t know anything about computing wall shear stress myself. There are several references for trying to do this:
- [How to get Wall Shear Stress on a model](https://discourse.paraview.org/t/how-to-get-wall-shear-stress-on-a-model/5694)
- [https://www.paraview.org/pipermail/paraview/2016-October/038305.html](https://www.paraview.org/pipermail/paraview/2016-October/038305.html)
- [[General] Calculate wall shear stress with paraview -- CFD Online Discussion Forums](https://www.cfd-online.com/Forums/paraview/235902-calculate-wall-shear-stress-paraview.html)
That said, it might be better to have Nek5000 compute the wall shear stress if you can. Several people reported problems with computing it post processing including inaccurate velocities on the surface due to boundary conditions.